The invention relates generally to rolls for treatment of webs of material and, more particularly to a process for reducing vibrations in sets of rolls, as well as to a roll arrangement comprising at least two immediately successive sets of rolls having reduced vibratory tendencies.
As used herein the expression "immediately successive" roll sets refers to a roll arrangement in which no additional sets of rolls are provided between two successive sets of rolls. However, guide rollers and the like may be provided between the immediately successive sets of rolls and, thus, this expression does not preclude provision of such additional rollers.
An arrangement of sets of rolls is disclosed in U.S. Pat. No. 4,534,829, which shows a double calendar having two successive sets of rolls. A roll arrangement having more than two successive sets of rolls is depicted in DE-OS 3119691. Such roll arrangements tend to vibrate under certain conditions especially when one or both of the rolls in a set are elastic, i.e., "soft", rolls. These vibrations are manifested not only in undesirable loud noises and vibratory loading of the system's components, but also in vibrations producing undesirable patterns on a web conducted through the nips formed between the roll sets.
Observations have revealed that the second roll set with respect to the direction of web travel generally experiences more vibrations than the first roll set. This phenomenon is attributed to the fact that a random excitation causes the first set of rolls to vibrate according to its natural frequency. While these vibrations may be very weak, they are manifested as successive irregularities in the web, which correspond to the rate of the vibrations. These irregularities constitute a systematic excitation for the second set of rolls when the second set is of a similar construction as the first set of rolls, which usually is the case, and thereby has the same natural frequency as that of the first set.
The vibration problem in calendars having abutting rolls arranged above each other is discussed in DE-AS 2420563. The solution proposed therein is provision of rolls having different diameters, which are selected according to a specific rule. However, in this solution all of the rolls must be situated on top of each other. The invention, on the other hand, is concerned with the vibration problems occurring in successive, separate sets of rolls.